The retreading of a tire consists in separating it, i.e., removal by a machining operation of what remains of the worn tread, then mounting a new tread. The retreading requires that a mechanical bond be formed between the materials of the crown of the tire and the new tread, whether or not it is prevulcanized.
Usually this mechanical bond is created by inserting a nonvulcanized cushion gum between the crown and the new tread.
Consequently, when retreading is done with a prevulcanized tread, it is necessary to obtain an excellent bond between, on the one hand, the (nonvulcanized) cushion gum and, on the other hand, the materials of the crown and the new (vulcanized) tread.
Likewise, when a tire is being repaired, an excellent bond must be obtained between the materials of the damaged areas, after their degraded parts are removed, and a nonvulcanized repair material that may or may not be accompanied by a reinforcing part or vulcanized "patch." To obtain good bonding, it is necessary, in particular, that the surfaces of the vulcanized materials be clean, that their oxidized or degraded surface parts be removed, that they have a suitable roughness, and that contact with the nonvulcanized material be made under good conditions with, in particular, sufficient application pressure.
In the case of retreading tires, the machining intended to remove what remains of the worn tread can be performed in several ways. A first method consists in using a "buffing machine." Such a machine uses a rotary abrasive tool including a set of metal disks having abrasive teeth on their outside circumferences. These disks are assembled perpendicular to a common axis of rotation. The buffing makes it possible to remove what remains of the worn tread and thus to bring the crown of the casing to the desired geometric shape while imparting to the rubber surface a roughness that is suitable for the subsequent bonding. An example of suitable roughness is provided by the reference plates provided for retreading and repairing tires by the Rubber Manufacturers Association. This machining by buffing which is simple and effective, is the process currently used.
However, it has several drawbacks: (1) a great deal of damaging heating of the rubber surface is caused; (2) a great deal of rubber debris is produced which needs to be vacuumed off; and (3) difficult work conditions are created because of the pollution due to the noise and the rubber crumb.
Another method consists in removing what remains of the worn tread with either hot or cold blades. However, after this machining, the rubber surfaces obtained are smooth; this necessitates a subsequent brushing operation to create the surface roughness that experiments have shown to be necessary for good bonding reliability.
A critical area of the mechanical bond between the crown of the tire and the new tread is the area located at the two lateral edges of the new tread. Depending on the geometry of the tread used (i.e. flat tread, flared tread, curved tread . . . ,), the lateral edges or flarings or "flanges" are connected to the edges of the crown or to the shoulders of the tire. This area is heavily stressed mechanically during driving, and it has been found that any bonding failure causes cracks to appear, and then to spread. These cracks can then spread along the interfaces between the cushion gum and the crown or the tread, and may also penetrate directly into the materials of the crown or the shoulders.